Underground supporters (5)
I believe the answer is:
roots
'underground supporters' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for roots that I've seen before include "authentic music" , "To find ancestry" , "Cultural origins" , "Searches vigorously" , "Underground parts of plants" .)